The present invention relates to an evaporation control method for liquid fuel in a fuel cell system, and more particularly to an evaporation control method for liquid fuel in an evaporator for preventing liquid accumulation, i.e., preventing liquid fuel from accumulating in liquid state in an evaporation chamber of the evaporator when power load is increased suddenly. The present invention also relates to a fuel cell system applying such a method.
A fuel cell system is an electric power generation system centered on a fuel cell generating electricity by providing an anode of the fuel cell with hydrogen as fuel gas and providing a cathode of the fuel cell with oxidation gas containing oxygen. The fuel cell system converts directly chemical energy into electrical energy and has high efficiency of electric power generation and an extremely low emission amount of a toxic substance, therefore, receives attention recently.
However, in the fuel cell system, from the viewpoint of difficulty of handling hydrogen and diversification of fuel, liquid fuel from, e.g., methane gas and methanol on down except for hydrogen is reformed by a reforming device and the reformed gas having generated hydrogen as the main ingredient is often used as fuel gas. It will take much time to fill with hydrogen a conveyance such as a vehicle on which the fuel cell system is mounted, e.g., a fuel cell electric vehicle. Therefore, it is inconvenient to use hydrogen. Moreover, it is difficult to increase an amount of hydrogen, which can be mounted, accordingly, there is a drawback that mileage is shortened.
Therefore, it is considered to fill a tank mounted on a vehicle with liquid fuel such as methanol to be reformed by reforming device and the reformed gas having generated hydrogen as the main ingredient to be used as fuel gas.
It can be carried out to fill a vehicle with methanol as with a refueling gasoline engine vehicle and mileage in this case bears comparison with that of a gasoline engine vehicle. Therefore, the vehicle in this case is handled as with a refueling gasoline engine vehicle.
As shown in FIG. 5, a fuel reforming apparatus of a conventional fuel cell system to be mounted on a vehicle is formed of an evaporator 100 for evaporating the liquid fuel, a reforming device 101 for making gas of the liquid fuel evaporated by the evaporator 100, react by a solid catalyst to form fuel gas, and a CO removing device 102 for removing carbon monoxide from the fuel gas generated by the reforming device 101. The conventional fuel cell system comprises the fuel reforming apparatus, a fuel cell 103 for making hydrogen in the fuel gas provided from the fuel reforming apparatus with oxygen in the air compressed by an air compressor of an oxidizing agent providing means to generate electric power, and a burner 104 with an auxiliary fuel supplying line for burning off-gas of an anode of the fuel cell 103 to generate gas to be a heating source of the evaporator 100.
However, there is a problem in the conventional art. When the vehicle accelerates or goes up a hill, a required load (electric power) of the fuel cell 103 is increased and a supply amount of the liquid fuel is increased suddenly at the same time. A load of the evaporator 100 is changed suddenly and supply of the off-gas to the burner 104 cannot reach the required amount immediately, therefore, there is a shortage of an amount of heat of the burner 104. Accordingly, the liquid fuel cannot be evaporated completely and there is liquid accumulation, i.e., the liquid fuel accumulates in liquid state in an evaporation chamber of the evaporator 100.
When there is liquid accumulation, even if supply of the liquid fuel to the evaporator 100 is cut off by a cut-off valve at lightening the load when the vehicle decelerates or goes down a hill, there is liquid fuel remaining in the evaporation chamber of the evaporator 100. Therefore, the liquid fuel continues to be evaporated and responsivity of the evaporator 100 is deteriorated. The evaporation gas is supplied to the reforming device 101 and reformed to hydrogen-rich gas to continue being supplied to the fuel cell 103. At decreasing the load, the fuel cell 103 cannot consume completely the fuel gas, therefore, an amount of off-gas is increased and it causes overheating of the burner 104 and deterioration of heating efficiency.
When the liquid fuel is a mixture, in a low temperature evaporation state in which there is liquid accumulation, an ingredient easy to evaporate is evaporated early because different materials coexist. Therefore, there is a problem that composition of gas at an outlet of the evaporator 100 exhibits variations. As a consequence, there are problems relating to control of an amount of air and temperature of the reforming device 101 and the CO removing device 102. It causes that there is a shortage of an amount of water vapor to be supplied to the fuel cell 103 and carbon monoxide cannot be removed completely, therefore, an electrode component of the fuel cell 103 is rendered poisonous and performance of a fuel reforming apparatus deteriorates.
Therefore, for operating a system having large load regulation such as a fuel cell electric vehicle with efficiency, the supply amount of the liquid fuel to the evaporator 100 need be controlled so as to prevent liquid accumulation in the evaporation chamber of the evaporator 100.
Conventionally, following methods for doing away with liquid accumulation are adapted.
(1) The burner 104 is supplied with auxiliary fuel and the auxiliary fuel is burned so that combustion gas is generated and an amount of heat of it makes up for a shortage of an amount of heat of the burner 104.
(2) An amount of air for supplying to the burner 104 is reduced (a ratio air to fuel is reduced) and an amount of heat carried out by exhaust gas from the burner 104 to the outside so that the temperature of combustion gas of the burner 104 is increased.
However, optimum control cannot be achieved by the above-described methods.
The present invention is provided for solving the above-described problems and an object of the present invention is to provide an evaporation control method for liquid fuel in an evaporator for controlling liquid accumulation of liquid fuel in an evaporation chamber of the evaporator.
To solve the above-described problem, a first aspect of the present invention provides an evaporation control method for liquid fuel in a fuel cell system, the fuel cell system comprising:
a fuel reforming apparatus formed of an evaporator for evaporating liquid fuel, a reforming device for making gas of the liquid fuel evaporated by the evaporator react by a solid catalyst to be hydrogen-rich fuel gas, and a CO removing device for removing carbon monoxide from the fuel gas generated by the reforming device;
a fuel cell for making hydrogen in the fuel gas provided from the fuel reforming apparatus react with oxygen provided from an oxidizing agent providing means to generate electric power; and
a burner for burning off-gas of an anode of the fuel cell to generate combustion gas to be a heating source of the evaporator,
in which a temperature detector for detecting temperature at a bottom of an evaporation chamber is provided in the evaporator and a supply amount of the liquid fuel to the evaporator is reduced and controlled from a supply amount (command value) corresponding to a required load amount of the fuel cell in accordance with temperature detected by the temperature detector.
The temperature detector is provided at the bottom of the evaporation chamber of the evaporator and the supply amount of the liquid fuel to the evaporator is reduced and controlled from the supply amount (a command value) of the liquid fuel corresponding to the required load amount of the fuel cell in accordance with temperature detected by the temperature detector. Therefore, liquid accumulation in the evaporation chamber of the evaporator can be prevented.
A second aspect of the present invention according to the first aspect provides an evaporation control method for liquid fuel in a fuel cell system in which the supply amount of the liquid fuel to the evaporator is reduced and controlled on the basis of the detected temperature at the bottom of the evaporation chamber and the liquid fuel is controlled and prevented from accumulating in liquid state in the evaporation chamber of the evaporator.
The supply amount of the liquid fuel to the evaporator is reduced and controlled on the basis of the detected temperature in the evaporation chamber of the evaporator, therefore, the liquid fuel can be controlled and prevented from accumulating in liquid state in the evaporation chamber of the evaporator.
The third aspect of the present invention according to the first or second aspect provides an evaporation control method for liquid fuel in a fuel cell system in which it is stopped to supply the liquid fuel to the evaporator on the basis of the detected temperature at the bottom of the evaporation chamber and a state in which the liquid fuel is accumulated in liquid state in the evaporation chamber of the evaporator is done away with.
It is stopped to supply the liquid fuel to the evaporator on the basis of the detected temperature at the bottom of the evaporation chamber of the evaporator, therefore, a state in which the liquid fuel is accumulated in liquid state in the evaporation chamber of the evaporator can be controlled to do away with.
The forth aspect of the present invention according to the first, second or third aspect provides an evaporation control method for liquid fuel in a fuel cell system in which temperature of the combustion gas of the burner is detected, auxiliary combustion is carried out or stopped in accordance with the detected temperature of the combustion gas and the supply amount of the liquid fuel to the evaporator is controlled with maintaining the temperature of the combustion gas supplied to the evaporator in a predetermined range.
The temperature of the combustion gas of the burner is detected, auxiliary combustion is carried out or stopped in accordance with the detected temperature of the combustion gas and the supply amount of the liquid fuel to the evaporator is controlled with maintaining the temperature of the combustion gas supplied to the evaporator in the predetermined range. Therefore, the combustion gas to be a heating source of the evaporator is secured with stability. Liquid accumulation in the evaporation chamber of the evaporator can be prevented and controlled more suitably in comparison with a case in which the supply amount of the liquid fuel to the evaporator is controlled on the basis of only the detected temperature at the bottom of the evaporation chamber.
The fifth aspect of the present invention according to the first, second, third or forth aspect provides an evaporation control method for liquid fuel in a fuel cell system in which when the supply amount of the liquid fuel to the evaporator is subjected to xe2x80x9creductionxe2x80x9d or xe2x80x9cstopxe2x80x9d control, output of the fuel cell is limited to an output value in accordance with the supply amount of the liquid fuel.
The output of the fuel cell is limited to the output value in accordance with the supply amount of the liquid fuel to the evaporator, therefore, the whole fuel cell system can be subjected to power control without laboring.